1. Field of the Invention
The present invention relates to controlled approach of a tool to a workpiece, and in a specific arrangement concerns a method of reducing the time required to sequentially place a tool onto a plurality of work sites in sequence, when the work site dimensions relative to a reference coordinate system vary by some acceptable tolerance from one work site to another.
2. Description of Related Art
In many types of repetitive sequences a tool is moved to each of a series of work sites on a part to operate on the individual work sites. The operation performed on the work site is usually a fixed short time duration, while the time required to properly and precisely position the tool onto the work site may be a significantly longer time. Thus, the total time of each operation sequence is greatly lengthened because of the time necessary to properly position the tool. This problem is worse where the tool must be precisely positioned with respect to the work site so that a slow speed search mode must be employed in approach of the tool to the work site.
The problem of time of approach of a tool to a work site is of particular importance in various types of automatically controlled bonding operations, including tape automated bonding, wedge bonding, and ball bonding. In such bonding operations it is common to perform operations on a number of closely adjacent work sites or pads, in sequence, by automatic control of a bonding tool. Generally a bonding tool under computer control is mounted on a carriage that moves horizontally in X and Y relative to a work table upon which the part containing the pads to be bonded is mounted. The bonding tool is first positioned directly over the pad to be bonded and then moved downwardly at a rapid rate to a search height, which is a tolerance height above the pad position. This is to ensure that the tool will not contact the pad while the tool is moving at a high velocity. Once at the tolerance height the tool is moved downwardly toward the pad at a very slow, constant search velocity, until contact (touchdown) between the bonding tool and the pad to be bonded is sensed. Motion of the carriage that compliantly mounts the bonding tool is continued, beyond touchdown, through a selected overtravel distance to ensure good contact and application of a predetermined force between the tool and the pad. Because the height of each pad varies from pad to pad, the tool is moved through its tolerance height zone using a slow speed search mode to approach each pad. The search mode is slow and time consuming and thus greatly adds to the time required for performing a plurality of bonding operations.
Accordingly, it is an object of the present invention to provide a method of moving a tool to a workpiece that avoids or minimizes above-mentioned problems.